Interest in organic semiconductor photo devices, such as organic photodetectors (PD) and photovoltaic cells (PV), has increased because such devices can now be inexpensively fabricated on any suitable substrate, such as flexible substrates or non-planar/curved substrates. However, due to the reduced mobility of charge carriers associated with organic semiconductors, current generation photo devices suffer from many drawbacks, including reduced operating performance. For example, key performance characteristics associated with the operation of photodetectors (PD) are responsitivity and noise, while the key performance characteristics associated with the operation of photovoltaic cells (PV) are short circuit current (Isc), open circuit voltage (Voc), and fill factor (FF). The responsitivity of photodetectors (PD), and similarly the short circuit current (Isc) of photovoltaic cells (PV), is determined by the amount of light that is permitted to be absorbed by the photo device, along with its charge carrier collection efficiency. In particular, the ability of a photo device to absorb light depends primarily on the thickness of the absorbing layer of the photo device. Alternatively, the carrier collection efficiency of the photo device depends on the diffusion length of the charge carriers relative to the distance from the location of the photo carrier generation to a collecting junction.
To achieve desired levels of light absorption and carrier collection efficiency, the standard method of making PIN photo detectors (PD) or photovoltaic (PV) cells, where a p-type material is used as an insulator or an n-type material is used as an absorbing layer, is not feasible. Specifically, to obtain a sufficient amount of light absorption, the absorbing layer of the photo device must be on the order of microns (μm) thick. Furthermore, the diffusion length of the photo device can be calculated by the formula L=(μτVt)1/2, where L is the diffusion length, Vt is the thermal voltage (0.0259 eV at room temperature), μ is the carrier mobility, and τ is the charge carrier lifetime. The diffusion length (L) for a typical charge carrier mobility of 10−3 cm2/Vsec and a typical carrier lifetime of 100 ns in organic materials is about 16 nm. Such a diffusion length (L) is substantially shorter than the distance from the location of the absorption layer to the collecting junction (e.g. approximately the micron thickness of the absorbing layer in a standard photo device). Thus, the thickness of the light absorbing layer required for photo devices of current designs to obtain sufficient light absorption is too long to provide satisfactory carrier collection efficiency.
Another problem associated with the fabrication of photo devices is noise, and specifically, in the case of photovoltaic (PV) devices, low open circuit voltage (Voc). For example, one of the major contributions to noise in photodetectors, and to low open-circuit voltages (Voc) in photovoltaic devices is dark current. Dark current results from either band-to-band leakage or from defect leakage in the photo device. In particular, band-to-band leakage depends on the bandgap, which is determined by the wavelength of the received light, while defect leakage is the result of surface/interface defects in the photo device. As such, defect leakage is the primary leakage current for most photo devices, including photodetector (PD) devices and photovoltaic (PV) devices. Carbon nanotubes, such as II-VI nanorods, typically have benign surface properties and are suited for non-crystalline applications. Thus, even though these non-crystalline organic materials have a substantial amount of interface defects, such defects are benign and do not contribute to carrier recombination. The defect leakage current is further reduced due to the low-charge carrier mobility, which tends to localize the effect of the leakage current associated with the defect.
Therefore, there is a need for a polymer photo device, such as a photodetector (PD) or photovoltaic cell (PV) that utilizes a light-absorbing material, which is able to absorb light over a wide spectral range, so as to provide broadband operation. In addition, there is a need for a polymer photo device, such as a photodetector (PD) or photovoltaic cell (PV) that utilizes a light-absorbing material, which enables increased carrier collection efficiency. Furthermore, there is a need for a polymer photo device, which has increased photo responsitivity or light absorption. In addition, there is a need for a polymer photo device, such as a photodetector (PD) or photovoltaic cell (PV) that utilizes a light-absorbing material that allows the photo device to have reduced dark current.